Process for preparing a film having alternating monolayers of a metal-metal bonded complex monolayer and an organic monolayer by layer-by layer growth

ABSTRACT

The present invention provides a process for preparing a thin film having alternating monolayers of a metal-metal bonded complex monolayer and an organic monolayer by layer-by-layer growth. The process comprises the steps of: (1) applying onto a surface of a substrate a first linker compound to produce a primer layer; (2) applying onto said primer layer a layer of a metal-metal bonded complex to produce a metal-metal bonded complex monolayer on said primer layer; (3) applying onto said metal-metal bonded complex monolayer a second linker compound; and optionally (4) sequentially repeating steps (2) and (3) at least once to produce said layer-by-layer grown thin film having alternating monolayers of a metal-metal bonded complex monolayer and an organic monolayer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing a thin filmhaving alternating monolayers of a metal-metal bonded complex monolayerand an organic monolayer by layer-by-layer growth. More particularly,the present invention relates to a thin film having alternatingmonolayers of a metal-metal bonded complex monolayer and an organicmonolayer.

2. Description of the Prior Art

During the past three decades, considerable progress has been made inthe understanding of dinuclear compounds containing multiple metal-metalbonds. Both the experimental and the theoretical aspects of thesecompounds have been explored extensively. These studies have provided alarge body of information particularly in the following areas: thereactivities of the dinuclear cores, the strengths of metal-metalinteractions, the electronic transitions between metal-based orbitalsand those involving metal to ligand charge transfer, the redoxactivities of the dinuclear core, and the correlation among theseproperties (See, e.g., Cotton, Walton, Multiple Bonds Between MetalAtoms, 2nd Ed., Oxford, 1993).

Efforts focusing on technologically important applications of dinuclearcompounds have led to many promising research areas, such as inorganicliquid crystals (See, e.g., Chisholm, Acc. Chem. Res., 2000, 33, 53),antitumor agents (See, e.g., Hall, et al, J. Clin. Hematol. Oncol.,1980, 10, 25), and homogeneous and photolytic catalysis (See, e.g.,Doyle, Aldrichimica Acta, 1996, 29, 3; Nocera, Acc. Chem. Res., 1995,28, 209).

Layer-by-layer assembly techniques to fabricate multicomponent films hasbeen explored in the literature. One of the most developed systems grownlayer-by-layer is the layered metal phosphates and phosphonates. Thefilms include multivalent metal ions, e.g. Zr⁴⁺, and organic moleculesterminated with an acidic functionality, e.g. a phosphonic acid (See,e.g., Cao, Hong, Mallouk, Acc. Chem. Res., 1992, 25, 420). Katz andco-workers have used this method to align hyperpolarizable moleculesinto polar multilayer films that show second-order nonlinear opticaleffects (See, e.g., U.S. Pat. Nos. 5,217,792 and 5,326,626). A similarapproach has also been extended to other materials such as polymers,natural proteins, colloids, and inorganic clusters (See, e.g., Decher,Science, 1997, 277, 1232). This same technique has also been applied tothe production of other multilayers including Co-diisocyanide, dithiolswith Cu, and pyrazines with Ru (See, e.g., Page, Langmuir, 2000, 16,1172).

Among the existing examples, the driving force for the film progressionis mainly the electrostatical interaction between polycations andpolyanions; few examples involve other types of interactions, such ashydrogen bond, covalent, or mixed covalent-ionic. The present inventionutilizes strong covalent interactions, rather than ionic interactions,between the metals and the ligands in a novel strategy to assemblenearly perfectly packed multilayers.

Despite the abundance of activity, efforts so far have been limited tothe study and use of the metal-metal bonded compounds in solution-basedsystems. To harness the electronic, optical, and magnetic properties ofmetal-metal bonded materials in solid-state applications and devices,development of new methods for making thin films containing functionalmetal-metal bonded complexes are needed.

Accordingly, the present invention provides methods to grow multilayerthin films including metal-metal bonded compounds.

SUMMARY OF THE INVENTION

The present invention provides a process for preparing a thin filmhaving alternating monolayers of a metal-metal bonded complex monolayerand an organic monolayer by layer-by-layer growth. The process includesthe steps of:

(1) applying onto a surface of a substrate a first linker compoundrepresented by the formula:

G1-Linker_(a)-G2

to produce a primer layer of said first linker compound on saidsubstrate, wherein G1 is a functional group capable of interacting withsaid surface of said substrate; G2 is a functional group capable ofinteracting with a metal-metal bonded complex; and Linker_(a) is adifunctional organic group bonded to G1 and G2;

(2) applying onto said primer layer a layer of a metal-metal bondedcomplex to produce a metal-metal bonded complex monolayer on said primerlayer; said metal-metal bonded complex being selected from the groupconsisting of compounds represented by the following formulas:

and a combination thereof; wherein:

L_(ax) is an axial ligand;

L_(eq) is an equatorial ligand; wherein two equatorial ligands togetherform a bidentate ligand

wherein each

is independently selected from the group consisting of:

M is a transition metal;

wherein

is a bridging group each selected independently from the groupconsisting of: SO₄ ²⁻, MoO₄ ²⁻, WO₄ ²⁻, ZnCl₄ ²⁻ and a dicarboxylate;and

wherein m is an integer from 1 to 25, and n is 0 to 6;

(3) applying onto said metal-metal bonded complex monolayer a secondlinker compound represented by the formula:

G3-Linker_(b)-G4

to produce on said metal-metal bonded complex monolayer an organicmonolayer; wherein G3 and G4 are the same or different functional groupscapable of interacting with a metal-metal bonded complex; and Linker_(b)is a single bond or a difunctional organic group bonded to G3 and G4;and optionally

(4) sequentially repeating steps (2) and (3) at least once to producesaid layer-by-layer grown thin film having alternating monolayers of ametal-metal bonded complex monolayer and an organic monolayer.

More particularly, the present invention provides a process forpreparing a thin film having alternating monolayers of a metal-metalbonded complex monolayer and an organic monolayer by layer-by-layergrowth. The process includes the steps of:

(1) applying onto a surface of a substrate a first linker compoundrepresented by the formula:

G1-Linker_(a)-G2

to produce a primer layer of said first linker compound; wherein G1 isselected from the group consisting of: Cl₃Si and SH; G2 is selected fromthe group consisting of: 4-pyridyl and 4-cyanophenyl; and Linker_(a) isselected from the group consisting of: C₁-C₈ alkylene, C₁-C₈ alkenediyl,C₁-C₈ alkynediyl and 1,4-arylene;

(2) applying onto said primer layer a metal-metal bonded complex toproduce on said primer layer a metal-metal bonded complex monolayer;wherein said metal-metal bonded complex is selected from the groupconsisting of compounds represented by the following formulas:

and a combination thereof; wherein:

L_(ax) is an axial ligand;

L_(eq) is an equatorial ligand; wherein two equatorial ligands togetherform a bidentate ligand

wherein each

is independently selected from the group consisting of:

M is a transition metal;

wherein the group

is a dicarboxylate bridging group selected from the group consisting ofcompounds represented by the formulas:

and mixtures thereof; and

wherein m is an integer from 1 to 12, and n is 0 to 3;

(3) applying onto said metal-metal bonded complex monolayer a secondlinker compound represented by the formula:

G3-Linker_(b)-G4

to produce on said metal-metal bonded complex monolayer an organicmonolayer; wherein G3 and G4 are the same or different functional groupscapable of interacting with a metal-metal bonded complex; and Linker_(b)is a single bond or a difunctional organic group bonded to G3 and G4;and optionally

(4) sequentially repeating steps (2) and (3) at least once to producesaid layer-by-layer grown thin film having alternating monolayers of ametal-metal bonded complex monolayer and an organic monolayer.

The present invention further provides a thin film having alternatingmonolayers of a metal-metal bonded complex monolayer and an organicmonolayer by layer-by-layer growth. Such films can be prepared by theprocesses according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. UV-VIS spectra of bilayer films as multilayers are assembledincreasing the film thickness from 1 to 10 bilayers. Inset: A. UV-visspectra of the first three bilayers (total six monolayers) showing thealternative adding of the metal-metal bonded compounds (at 258 nm) andthe organic species (at 296 nm). B. Increase in absorbance at 258 nm asa function of the number of added bilayers.

FIG. 2. AFM measurements of thin film thickness and corresponding imagesof thin films including of 5, 10, 15, and 20 bilayers.

FIG. 3. AFM images of thin films grown on thermally oxidized Si wafersincluding of 5 and 20 bilayers.

FIG. 4. Cyclic voltammograms of one bilayer of the metal-metal bondedcompound [Rh₂(DAniF)₂]₂(O₂CCH₂CO₂)₂(NC₅H₄CHCHC₅H₄N)₄ on a modified ITOelectrode.

DETAILED DESCRIPTION OF THE INVENTION

The processes described herein include layer-by-layer growth of thinfilms having alternating monolayers of metal-metal bonded complexes andorganic molecules. Such films have utility in solid-state applications.

The films are prepared by repeated sequential depositions of metal-metalbonded units, e.g., dirhodium tetraformamidinate compounds, on aprefunctionalized substrate, followed by a proper organic linker, e.g.,dipyridyl organic molecules, for the next deposition sequence.

The deposition method is a self-assembling, tunable and stepwiseprocess. Upon application onto a substrate, the complexes are adsorbedon the substrate. Thereafter, an organic monolayer is applied. Thus,repeating the steps, a stepwise layer by layer growth of the thin filmscan be achieved.

The multi-layered thin films can be grown layer-by-layer to the desiredthickness. The process comprises the following steps:

(1) applying onto a surface of a substrate a first linker compoundrepresented by the formula:

G1-Linker_(a)-G2

to produce a primer layer of the first linker compound on the substrate,wherein G1 is a functional group capable of interacting with the surfaceof the substrate; G2 is a functional group capable of interacting with ametal-metal bonded complex; and Linker_(a) is a difunctional organicgroup bonded to G1 and G2;

(2) applying onto the primer layer a layer of a metal-metal bondedcomplex to produce a metal-metal bonded complex monolayer on the primerlayer; the metal-metal bonded complex being selected from the groupconsisting of compounds represented by the following formulas:

and a combination thereof; wherein:

L_(ax) is an axial ligand;

L_(eq) is an equatorial ligand; wherein two equatorial ligands togetherform a bidentate ligand

wherein each

is independently selected from the group consisting of:

M is a transition metal;

wherein

is a bridging group each independently selected from the groupconsisting of: SO₄ ²⁻, MoO₄ ²⁻, WO₄ ²⁻, ZnCl₄ ²⁻ and a dicarboxylate;and

wherein m is an integer from 1 to 25, and n is 0 to 6;

(3) applying onto the metal-metal bonded complex monolayer a secondlinker compound represented by the formula:

G3-Linker_(b)-G4

to produce on the metal-metal bonded complex monolayer an organicmonolayer; wherein G3 and G4 are the same or different functional groupscapable of interacting with a metal-metal bonded complex; and Linker_(b)is a single bond or a difunctional organic group bonded to G3 and G4;and optionally

(4) sequentially repeating steps (2) and (3) at least once to producethe layer-by-layer grown thin film having alternating monolayers of ametal-metal bonded complex monolayer and an organic monolayer.

The length, functionality, direction of metal-metal vector, and otherphysical and chemical properties of each layer can be tuned by varyingthe metal-metal bonded units and the organic linkers. Preferably, thethin film has from 1 to 60 alternating monolayers of a metal-metalbonded complex monolayer and an organic monolayer. More preferably, thethin film has from 30 to 40 alternating monolayers of a metal-metalbonded complex monolayer and an organic monolayer.

The films are deposited from liquid solutions and therefore they may bedeposited on substrates having diverse topography and configuration.

The following illustration describes the layer-by-layer growth methodsused according to the present invention to fabricate metal-metal bondedcompounds on a substrate.

As a substrate, any suitable material can be used. Suitable substratesinclude, for example, a metal, a metal oxide, a semiconductor, a metalalloy, a semiconductor alloy, a polymer, an organic solid, and acombination thereof. The form of the substrates can be a planar solid ora non-planar solid such as a stepped or curved surface.

The following preferred substrates have been demonstrated: Au, ITO andSiO₂.

G1-Linker_(a)-G2 groups are suitable molecular species that can form aself-assembled monolayer including organic molecular species having afunctional group G1 capable of interaction with the surface of thesubstrate forming a coated surface.

Examples of this group that can be designed into molecules forinteracting with or binding to a particular substrate surface withchemical specificity include one or more of the same or differentfunctional groups, such as phosphine oxide, phosphite, phosphate,phosphazine, azide, hydrazine, sulfonic acid, sulfide, disulfide,aldehyde, ketone, silane, germane, arsine, nitrile, isocyanide,isocyanate, thiocyanate, isothiocyanate, amide, alcohol, selenol, nitro,boronic acid, ether, thioether, carbamate, thiocarbamate,dithiocarbamate, dithiocarboxylate, xanthate, thioxanthate,alkylthiophosphate, dialkyldithiophosphate or a combination thereof.

Functional group G2 on the tran direction of G1 is capable ofinteraction with the next layer metal-metal boned molecules. Examples ofthis group that can be designed into molecules for interacting with orbinding to a particular metal-metal bonded molecule with chemicalspecificity include one or more of the same or different functionalgroups. Thus, G2 in the first linker compound can independently be:4-pyridyl, 3-pyridyl, cyano, 4-cyanophenyl, 3-cyanophenyl,perfluoro-3-cyanophenyl and perfluoro-4-cyanophenyl.

There are two types of these molecules, namely G2a and G2b in thedisclosure for the direction definition. G2a is used for the axialdirection linkage, such as nitrile, pyridyl, trimethylsilane compounds;and the G2b is used for the equatorial direction linkage, such as somebridging bidentate ligands with (N,N), (N,O), (O,O), (O,S), (P,P),(N,S), and (S,S) donor sets. Some typical examples of bidentate ligandsare amidinates that are a (N,N) donor set, acetamides that are a (N,O)set, carboxylates that are a (O,O) set, thiocarboxylates that are a(O,S) set, diphosphines that are a (P,P) set, mercaptopyrimidines thatare a (N,S) set, and dithiocarboxylates that are a (S,S) set.

The following molecules have been demonstrated:

-   -   on oxides surfaces, and

-   -   -   on Au surface.

III.

are suitable molecules containing at least one metal-metal bonded unit.

1. If the first monolayer ends with G2a group, examples of thesemetal-metal bonded compounds can be containing one or more than onemetal-metal bonded units of which axial direction can interact with orbind to G2a group, such as the molecules containing one or more than oneof the following metal-metal bonded cores: Cr₂ ⁴⁺, MO₂ ⁴⁺, Re₂ ⁶⁺, Re₂⁵⁺, Re₂ ⁴⁺, Ru₂ ⁵⁺, Ru₂ ⁶⁺, Rh₂ ⁴⁺. Preferred molecules suitable for useas the molecular species that can interact with or bind to G2a groupinclude: tetrakis(carboxylato)dichromium,tetrakis(carboxylato)dimolybdenum, tetrakis(amidinato)dichlorodirhenium,tetrakis(amidinato)chlorodiruthenium, tetrakis(carboxylato)dirhodium,tetrakis(amidinato)dirhodium, bis(carboxylato)bis(amidinato)dirhodium,and compounds containing more than one dimetal units.

If the first monolayer ends with G2b group, examples of thesemetal-metal bonded compounds can be containing one or more than onemetal-metal bonded units of which equatorial direction can interact withor bind to G2b group, such as the molecules containing one of thefollowing metal-metal bonded cores: Cr₂ ⁴⁺, Mo₂ ⁴⁺, W₂ ⁴⁺, Re₂ ⁶⁺, Re₂⁵⁺, Re₂ ⁴⁺, Ru₂ ⁴⁺, Ru₂ ⁵⁺, Ru₂ ⁶⁺, Os₂ ⁶⁺, Rh₂ ⁴⁺. Preferred moleculessuitable for use as the molecular species that can interact with or bindto G2b group include: tetrakis(carboxylato)dimetal (where the metal isthe one of the above), decakis(acetonitrile)dimetal (where the metal isMo, Re, and Rh).

The molecule that has been demonstrated is:[Rh₂(cis-N,N′-di-p-anisylformamidinate)₂]₂(O₂CCH₂CO₂)₂.

are suitable molecules bearing two functional groups at both ends. Thesefunctional groups will interact with or bind to the previous metal-metalbonded unit terminated surface. Both G3 and G4 functional groups areevery similar to G2.

Thus, G3 and G4 in the second linker compound can independently be4-pyridyl, 3-pyridyl, cyano, 4-cyanophenyl, 3-cyanophenyl,perfluoro-3-cyanophenyl and perfluoro-4-cyanopheny. Linker_(b) can be asingle bond, an alkylene, an alkenediyl, an alkynediyl, a 1,4-arylene,an arene-1,3,5-triyl, a 1,2,3-triazine-2,4,6-triyl,4,4′,4″,4′″-(21H,23H-porphine-5,10,15,20-tetrayl) and zinc complex of4,4′,4″,4′′-(21H,23H-porphine-5,10,15,20-tetrayl) and a combinationthereof. Further examples of G3-Linker_(b)-G4 groups includepolynitriles, polypyridyls, ditrimethylsilanes, and organic moleculescontaining at least two of any of the following donor sets used asbridging bidentate ligands: (N,N), (N,O), (O,O), (O,S), (P,P), (N,S),and (S,S), such as,

Some molecules with tetrahedral geometry may also be used as equatoriallinkers, such as SO₄ ²⁻, MoO₄ ^(*2−), WO₄ ²⁻, ZnCl₄ ²⁻.

Examples of the second linker compounds include compounds represented bythe following formulas:

and acetylene or diacetylene linkers represented by the formulas:

—C≡C— or —C≡C—C≡C—

which can be derived from derived from compounds represented by theformula:

Me₃Si—C≡C—SiMe₃

or

Me₃Si—≡—≡—SiMe₃

by desilylation of the trimethylsilyl group.

Preferred molecules carrying at least two required functional groupsinclude:

In a preferred embodiment, the process of the present invention includesthe steps of:

(1) applying onto a surface of a substrate a first linker compoundrepresented by the formula:

G1-Linker_(a)-G2

to produce a primer layer of the first linker compound; wherein G1 isselected from the group consisting of: Cl₃Si and SH; G2 is selected fromthe group consisting of: 4-pyridyl and 4-cyanophenyl; and Linker_(a) isselected from the group consisting of: C₁-C₈ alkylene, C₁-C₈ alkenediyl,C₁-C₈ alkynediyl and 1,4-arylene;

(2) applying onto the primer layer a metal-metal bonded complex toproduce on the primer layer a metal-metal bonded complex monolayer;wherein the metal-metal bonded complex is selected from the groupconsisting of compounds represented by the following formulas:

and a combination thereof; wherein:

L_(ax) is an axial ligand;

L_(eq) is an equatorial ligand; wherein two equatorial ligands togetherform a bidentate ligand

wherein each

is independently selected from the group consisting of:

M is a transition metal;

wherein the group

is a dicarboxylate bridging group selected from the group consisting ofcompounds represented by the formulas:

and mixtures thereof; and

wherein m is an integer from 1 to 12, and n is 0 to 3;

(3) applying onto the metal-metal bonded complex monolayer a secondlinker compound represented by the formula:

G3-Linker_(b)-G4

to produce on the metal-metal bonded complex monolayer an organicmonolayer; wherein G3 and G4 are the same or different functional groupscapable of interacting with a metal-metal bonded complex; and Linker_(b)is a single bond or a difunctional organic group bonded to G3 and G4;and optionally

(4) sequentially repeating steps (2) and (3) at least once to producethe layer-by-layer grown thin film having alternating monolayers of ametal-metal bonded complex monolayer and an organic monolayer.

In the first step, the substrates used for film growth can be variouskinds of metals, insulators, and semiconductors such as glass, quartz,aluminum, gold, platinum, gold/palladium alloy, silicon, thermally grownsilicon dioxide on silicon, and indium-tin-oxide coated glass. Since thefilms are deposited from liquid solutions, they may be deposited onsubstrates having diverse topography and configuration. The form of thesubstrates can be a planar solid or a non-planar solid such as a steppedor curved surface.

The second step of thin film deposition is to treat the modifiedsubstrate with an appropriate compound containing at least onemetal-metal bonded unit from solution. Metal-metal bond units willinteract with N atoms through their axial directions or with bidentateligands through their equatorial directions. The opposite direction thathas not been used to interact with the molecular template will be usedas the site for the next step of the layer-by-layer thin film growth.The metal atoms used in the metal-metal bonded units may be any of thefollowing: V, Nb, Cr, Mo, W, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt,Cu, Ag.

In the third step, the metal-metal bond unit terminated surface istreated with a solution containing molecules bearing at least twofunctional groups.

For axial linking these functional groups may be any kind of nitrile orpyridyl containing N-donor atoms. Thus, the organic molecules will bepolypyridyls, polynitriles, or will contain both pyridyl and nitrilefunctionalities.

For equatorial linkers, they can be organic molecules containing atleast two of any of the following donor sets used as bridging bidentateligands: (N,N), (N,O), (O,O), (N,P), (P,P), (N,S), and (S,S). Somemolecules with tetrahedral geometry may also be used as equatoriallinkers, such as SO₄ ²⁻, MoO₄ ²⁻, WO₄ ²⁻, ZnCl₄ ²⁻.

The next step is to repeat the above two steps to add additional layers,but the metal-metal bonded units and organic linkers are not required tobe the same, as long as they have a similar structural moiety. Thisprovides a versatile means of assembling multilayer heterostructuresfrom various metal-metal bonded building blocks, with essentially anydesired sequence of layers.

The scheme below illustrates an example of multilayer thin film growthincluding of alternating layers of the redox active metal-metal bondedsupramolecules [Rh₂(DAniF)₂]₂(O₂CCH₂CO₂)₂(DAniF=N,N′-di-p-anisylformamidinate), 1, andtrans-1,2-bis(4-pyridyl)ethylene, 2, on pyridyl functionalized oxidesubstrates, such as quartz, indium-tin-oxide (ITO), and silicon wafersthat have a native or thermally grown silicon dioxide surface.

The oxide substrates were cleaned as follows: each substrate was firsttreated in UV/ozone for 30 min., then rinsed thoroughly with acetone,dichloromethane, and water, and then dried in an oven at 120° C. for atleast 2 h. The substrate was treated again in UV/ozone for another 30min. right before film deposition.

Substrates were first silated by immersion in a toluene solutioncontaining 1 mM 4-[2-(trichlorosilyl)]-ethylpyridine for 30 min. Afterrinsing with copious amounts of toluene and ethanol, the substrates werevacuum-dried. Metal-metal bonded molecular films were grown by firstdipping the substrates into a 0.1 mM toluene solution of molecule 1 for2 h at −15° C. and then in a 0.1 mM ether solution of 2 for 30 min atroom temperature, with rinsing between steps.

After the first bilayer was deposited, the procedure was repeated, butwith the soaking time reduced to 1 min for each solution, until thedesired number of bilayers had been obtained.

These steps can be schematically represented as follows:

FIG. 1 illustrates the monitoring of film growth by UV-VIS spectroscopy.There are two noticeable bands at 258 and 296 nm for these bilayerfilms. The linear increase in absorbance as a function of the number ofbilayers indicates that the same amount of material is being depositedin each dipping cycle. A linear least-squares regression of theabsorbance data at 258 nm for 10 bilayers, as can be seen in the insetof FIG. 1, gives a slope of 0.010 absorbance unit per bilayer with theregression coefficient of 0.998. In order to calculate the surfacecoverage of the film, we assume that the solution ε_(max) values are thesame as the extinction coefficient for the two-dimensional layer. Basedon this assumption the calculated average bilayer coverage of the filmis about 1.1×10⁻¹⁰ mol/cm². This value is consistent with that of1.0×10⁻¹⁰ mol/cm² calculated for dense packing of molecules using thelattice constants from the single crystal X-ray structure solution.Thus, the film formation here is close to the tight packing of moleculesas measured in the bulk solid. The formation of the bilayer films wasalso monitored directly by atomic force microscopy. Predesigned sharpedge steps were made in silicon dioxide by etching steps into the oxidein a pattern defined by a photoresist mask.

FIG. 2 shows the increase in the film thickness as a function of thenumber of bilayers, together with its corresponding AFM pictures nearthe step edge. The linear regression fit gives a slope of 1.68 nm,corresponding to the average bilayer thickness, and an interception of0:6 nm, corresponding to thickness of silane monolayer. In the singlecrystal X-ray structure, the length of each bilayer is about 1.61 nm.Thus, our results suggest that the metal-metal bonded layers grow nearlyperpendicular to the substrate surface.

FIG. 3 shows the images of 5- and 20-bilayer films on oxidized Siwafers. The uniformity and morphology of the prepared thin films havebeen evaluated by atomic force microscopy. The films in both casesproduce a roughnesses of about 0.6 nm. These numbers are much less thana monolayer thickness. With both UV-vis spectra and AFMcharacterization, we conclude that the structure of the bilayer films onthe oxide surfaces is similar to the tightly packed bulk solid-statesingle crystal structure.

FIG. 4 shows the results of careful examination of thin films grown onITO surfaces by cyclic voltammetry in 1M NaCl aqueous solution. Theoxidation potential of Rh₂ ⁴⁺ to Rh₂ ⁵⁺ in bilayer films occurs at 220mV, according to E_(1/2)=(E_(pa)+E_(pc))/2. This value is close to thatfound for compound 1 in the CH₂Cl₂ solution with a Pt working electrode(310 mV). The average separation of E_(pa) and E_(pc) is 88 mV whilevarying the scan rate from 10 to 1000 mV/s. The peak currents show alinear dependence upon the scan rate. All these results indicate thatthe metal-metal bonded bilayer films are surface-bound, reversibly redoxactive species as in the bulk analog.

The present invention has been described with particular reference tothe preferred embodiments. It should be understood that variations andmodifications thereof can be devised by those skilled in the art withoutdeparting from the spirit and scope of the present invention.Accordingly, the present invention embraces all such alternatives,modifications and variations that fall within the scope of the appendedclaims.

1. A film having alternating monolayers of a metal-metal bonded complexmonolayer and an organic monolayer by layer-by-layer growth, prepared bythe process comprising: (1) applying onto a surface of a substrate afirst linker compound represented by the formula:G1-Linker_(a)-G2 to produce a primer layer of said first linkercompound; wherein G1 is selected from the group consisting of: Cl₃Si andSH; G2 is selected from the group consisting of: 4-pyridyl and4-cyanophenyl; and Linker_(a) is selected from the group consisting of:C₁-C₈ alkylene, C₁-C₈ alkenediyl, C₁-C₈ alkynediyl and 1,4-arylene; (2)applying onto said primer layer a metal-metal bonded complex to produceon said primer layer a metal-metal bonded complex monolayer; whereinsaid metal-metal bonded complex is selected from the group consisting ofcompounds represented by the following formulas:

and a combination thereof; wherein: L_(ax) is an axial ligand; L_(eq) isan equatorial ligand; wherein two equatorial ligands together form abidentate ligand

wherein each

is independently selected from the group consisting of

M is a transition metal; wherein the group

is a dicarboxylate bridging group selected from the group consisting ofcompounds represented by the formulas:

and mixtures thereof; and wherein m is an integer from 1 to 12, and n is0 to 3; (3) applying onto said metal-metal bonded complex monolayer asecond linker compound represented by the formula:G3-Linker_(b)-G4 to produce on said metal-metal bonded complex monolayeran organic monolayer; wherein G3 and G4 are the same or differentfunctional groups capable of interacting with a metal-metal bondedcomplex; and Linker_(b) is a single bond or a difunctional organic groupbonded to G3 and G4; and optionally (4) sequentially repeating steps (2)and (3) at least once to produce said layer-by-layer grown thin filmhaving alternating monolayers of a metal-metal bonded complex monolayerand an organic monolayer.
 2. The film of claim 1, wherein saidtransition metal in said metal-metal bonded complex is selected from thegroup consisting of: Cr₂ ⁴⁺, Mo₂ ⁴⁺, Re₂ ⁴⁺, Re₂ ⁵⁺, Re₂ ⁴⁺, Ru₂ ⁵⁺, Ru₂⁶⁺, Rh₂ ⁴⁺ and a combination thereof.
 3. The film of claim 1, whereinsaid first linker compound is selected from the group consisting of acompound represented by the formula:

for oxides surfaces and a compound represented by the formula:

for Au surfaces.
 4. The film of claim 1, wherein said L_(ax) axialligand is selected from the group consisting of: acetonitrile, halide,DMSO, H₂O, ethanol, methanol and a combination thereof.
 5. The film ofclaim 1, wherein said

ligand is an amidinate; said

is acetamide; said

is mercaptopyrimidine; said

bidentate ligand is a thiocarboxylate; said

bidentate ligand is a carboxylate; said

bidentate ligand is a diphosphine; and said

bidentate ligand is a dithiocarboxylate.
 6. The film of claim 1, whereinsaid metal-metal bonded complex is represented by the formula:

wherein said bidentate ligand

is selected from the group consisting of:

L_(ax) is acetonitrile, halide, DMSO, H₂O, ethanol, methanol; and M isselected form the group consisting of: Cr, Mo, Re, Ru, Rh.
 7. The filmof claim 1, wherein G1 is functional group selected from the groupconsisting of: phosphine oxide, phosphite, phosphate, phosphazine,azide, hydrazine, sulfonic acid, sulfide, disulfide, aldehyde, ketone,silane, germane, arsine, nitrile, isocyanide, isocyanate, thiocyanate,isothiocyanate, amide, alcohol, selenol, nitro, boronic acid, ether,thioether, carbamate, thiocarbamate, dithiocarbamate, dithiocarboxylate,xanthate, thioxanthate, alkylthiophosphate, dialkyldithiophosphate and acombination thereof.
 8. The film of claim 1, wherein each G2 in saidfirst linker compound is independently selected from the groupconsisting of: 4-pyridyl, 3-pyridyl, cyano, 4-cyanophenyl,3-cyanophenyl, perfluoro-3-cyanophenyl and perfluoro-4-cyanopheny. 9.The film of claim 1, wherein each G3 and G4 in said second linkercompound is independently selected from the group consisting of:4-pyridyl, 3-pyridyl, cyano, 4-cyanophenyl, 3-cyanophenyl,perfluoro-3-cyanophenyl and perfluoro-4-cyanopheny; and Linker_(b) isselected from the group consisting of: a single bond, an alkylene, analkenediyl, an alkynediyl, a 1,4-arylene, an arene-1,3,5-triyl, a1,2,3-triazine-2,4,6-triyl,4,4′4″,4′″-(21H,23H-porphine-5,10,15,20-tetrayl) and zinc complex of4,4′,4″,4′″-(21H,23H-porphine-5,10,15,20-tetrayl) and a combinationthereof.
 10. The film of claim 1, wherein said second linker compound isselected from the group consisting of a compound represented by theformula:


11. The film of claim 1, wherein said second linker compound is derivedfrom a compound represented by the formula:Me₃Si—C≡C—SiMe₃Me₃Si—≡—≡—SiMe₃ by desilylation of the trimethylsilyl group to producean acetylene or diacetylene linker represented by the formula:—C≡C— or —C≡C—C≡C—
 12. The film of claim 1, wherein said metal-metalbonded complex is selected from the group consisting of compoundsrepresented by the following formulas:

and a combination thereof; wherein

is a bidentate ligand; wherein the group

is a dicarboxylate: bridging group selected from the group consisting ofcompounds represented by the formulas:

and mixtures thereof; and wherein m is an integer from 1 to 12, and n is0 to
 3. 13. The film of claim 1, wherein said metal-metal bonded complexis represented by the formula:

wherein

is N,N′-di-p-anisylformamidinate ligand; and wherein the group

is a dicarboxylate bridging group selected from the group consisting ofcompounds represented by the formulas:


14. The film of claim 1, wherein said substrate is selected from thegroup consisting of: a metal, a metal oxide, a semiconductor material, ametal alloy, a semiconductor alloy, a polymer, an organic solid and acombination thereof.
 15. The film of claim 14, wherein said substrate isselected from the group consisting of: Au, ITO and SiO₂.
 16. The film ofclaim 1, which has from 1 to 60 alternating monolayers of a metal-metalbonded complex monolayer and an organic monolayer.